MySQL 5.5 Reference Manual

12.3.5.2. LOCK TABLES and Triggers

If you lock a table explicitly with LOCK
TABLES, any tables used in triggers are also locked
implicitly:

The locks are taken as the same time as those acquired
explicitly with the LOCK
TABLES statement.

The lock on a table used in a trigger depends on whether the
table is used only for reading. If so, a read lock suffices.
Otherwise, a write lock is used.

If a table is locked explicitly for reading with
LOCK TABLES, but needs to be
locked for writing because it might be modified within a
trigger, a write lock is taken rather than a read lock.
(That is, an implicit write lock needed due to the table's
appearance within a trigger causes an explicit read lock
request for the table to be converted to a write lock
request.)

Suppose that you lock two tables, t1 and
t2, using this statement:

LOCK TABLES t1 WRITE, t2 READ;

If t1 or t2 have any
triggers, tables used within the triggers will also be locked.
Suppose that t1 has a trigger defined like
this:

CREATE TRIGGER t1_a_ins AFTER INSERT ON t1 FOR EACH ROW
BEGIN
UPDATE t4 SET count = count+1
WHERE id = NEW.id AND EXISTS (SELECT a FROM t3);
INSERT INTO t2 VALUES(1, 2);
END;

The result of the LOCK TABLES
statement is that t1 and
t2 are locked because they appear in the
statement, and t3 and t4
are locked because they are used within the trigger:

t1 is locked for writing per the
WRITE lock request.

t2 is locked for writing, even though the
request is for a READ lock. This occurs
because t2 is inserted into within the
trigger, so the READ request is converted
to a WRITE request.

t3 is locked for reading because it is
only read from within the trigger.

t4 is locked for writing because it might
be updated within the trigger.